This invention relates to a throttling pintle-type fuel injection nozzle for internal combustion engines, and more particularly to improvements in the pintle of the fuel injection nozzle.
Throttling pintle-type fuel injection nozzles are generally used in internal combustion engines equipped with precombustion chambers or ones equipped with swirl chambers and have a reduced injection rate at the initial stage of injection so as to obtain smooth starting of combustion and thereby attain soft combustion within the combustion chambers of the engine.
A typical throttling pintle-type fuel injection nozzle comprises a nozzle body fixed to the nozzle holder of a fuel injection valve, and a nozzle needle slidably mounted within the nozzle body and liftable by the force of pressurized fuel. The nozzle needle has a seating portion disposed for seating on a valve seating portion of the nozzle body, and a cylindrical pintle formed on tip of the seating portion and slidably fitted through a nozzle hole formed in the nozzle hole.
The pintle and the nozzle hole cooperate to define therebetween a small annular gap for producing a fuel-throttling effect such that the fuel injection quantity is restrained at the start of lifting of the nozzle needle, i.e. at the beginning of fuel injection, thereby preventing a sudden increase in the pressure within the combustion chamber of the engine and accordingly preventing knocking of the engine.
However, such throttling pintle-type fuel injection nozzle has the disadvantage that hot combustion gases are intruded into the above annular gap between the pintle and the nozzle hole to cause decomposition of a film of fuel stuck on the inner peripheral surface of the nozzle hole to form carbon deposits. After a long period of use of the fuel injection nozzle, increased carbon deposits on the inner peripheral surface of the nozzle hole will almost fully clog the annular gap between the pintle and the nozzle hole, to reduce the throttling effect of the annular gap, causing knocking of the engine as well as deterioration of the emission characteristics of the engine due to incomplete combustion. If a measure is taken to restrain a rise in the temperature in the vicinity of the nozzle hole so as to minimize decomposition of the fuel film, sulfur contained in the fuel forms sulfuric acid which causes corrosion of a portion of the end wall surface of the nozzle body in the vicinity of the nozzle hole opening therein.
To overcome the above disadvantages, a fuel injection nozzle has been proposed in Japanese Provisional Patent Publication No. 57-49063. According to this proposed fuel injection nozzle, the nozzle hole has a conical shape with its inner diameter gradually decreasing toward its end opening in the combustion chamber so as to prevent intrusion of hot combustion gases into the annular gap between the nozzle hole and the pintle of the nozzle needle. The clearance between the nozzle needle and the above end of the nozzle hole assumes a very small value from 0 to 0.005 mm when the nozzle is in its closed position. However, it is very difficult to bore such a nozzle hole which has a very small diameter of the order of 1 mm, in such a manner that the inner diameter gradually decreases toward the open end, inviting low productivity as well as an increased manufacturing cost.